Uplink Logical Channel in GSM
1. Random access channel (RACH)
It is an uplink channel used for MS randomly access to network by requesting for an SDCCH. The request includes a 3bit setup reason (call request, paging response, location update request and short message request etc.) and a 5bit random reference number for MS to differentiate the access granted messages.
2. Stand-Alone Dedicated Control Channel (SDCCH)
It is a bi-directional dedicated channel used in transmission of signaling messages concerned with connection setup, location update message, short message, authentication message, encryption command, channel allocation message and various kinds of additional services etc. It can be divided into the Stand-Alone Dedicated Control Channel (SD/8) and the Dedicated Control Channel in combination with CCCH (SD/4).
3. Slow associated control channel (SACCH)
It is used together with the traffic channel or SDCCH. It carries some specific information while transmitting the subscriber information. At the uplink, it mainly transmits the measurement report ; while at the downlink, it mainly transmits some system information. These messages include the quality of communication, LAI, CELL ID, BCCH signal strength of the adjacent cell, NCC permit, cell option, TA and power control level etc.
4. Fast associated control channel (FACCH)
It is used together with TCH for providing signaling messages whose speed and timeliness are much higher than the slow associated control channel (SACCH) for the system during the transmission process. This channel uses frames borrowed from the traffic channel for its connection and transmits such instruction messages as “handover”. For the voice decoder can repeat the voice of the last 20ms, this kind of interruption due to frame stealing will not be detected by the subscriber. Besides the three types of control channels described above, there is a cell broadcast control channel (CBCH). It is used at the downlink and carries the short message service cell broadcast (SMSCB) information. This kind of control channel uses the same physical channel as that used in SDCCH.